This invention pertains to flywheel energy storage systems used for prevention of power interruptions, and more particularly a flywheel uninterruptible power supply for reliably storing several kilowatt-hours of energy at low cost. The flywheel system uses a solid steel alloy cylinder for a flywheel, which is supported for high-speed rotation using simple passive radial magnetic bearings. Use of a solid flywheel construction with a restricted diameter, steel alloy composition and a fracture mechanics design approach allows for increased operating speed and achievement of energy densities far above those previously obtained with large steel flywheels. This increased energy storage capability makes the flywheel commercially viable for use in back up power applications, significantly reducing the cost of the total flywheel system. A process for manufacture of the flywheel cylinders is disclosed which provides quality assurance of flywheel integrity.
Flywheel uninterruptible power supplies are becoming recognized as potentially viable economic alternatives to electrochemical batteries for prevention of power interruptions to critical loads. Electrochemical batteries used in these applications, and valve regulated lead acid batteries in particular, have many undesirable traits. The life of batteries is short, typically between 1 to 7 years depending on the environment and use. They require frequent periodic maintenance and inspection, are subject to thermal degradation and can fail unpredictably. Lead acid batteries and other types as well are also environmentally noxious. However, lead acid batteries are relatively inexpensive. Flywheel systems are thought to have promise to eliminate the disadvantages of batteries with the expectation of achieving 20 year lives with minimal or no maintenance, temperature insensitivity, previously unachievable reliability while being environmentally benign.
Flywheel uninterruptible power supplies use a rotating flywheel to store energy kinetically. A high-speed flywheel stores electrical energy in the rotating inertia of a flywheel. An attached motor/generator is used to accelerate and decelerate the flywheel for storing or retrieving energy. Flywheels can be either constructed of metal or of high strength composite materials. The flywheel can be supported for rotation on mechanical bearings, magnetic bearings or a combination. To reduce the losses from aerodynamic drag, the housing surrounding the flywheel can be maintained at a low pressure, or for slower flywheels it can be filled with a gas of small molecule size such as helium. Many designs of motor/generators exist and can be employed. Motor/generators can also be made as separate components.
Flywheel systems can be divided into two basic categories based on their desired function: power ride-through and energy back-up. A ride-thru system is typically designed for discharging a high level of power for a short duration of time until an auxiliary energy generating means such as a generator set can be brought online. Discharge times range from 10 seconds to about 2 minutes with power levels of up to several hundred kilowatts. Applications for ride-through flywheel uninterruptible power supplies include computer data centers and also critical manufacturing operations such as semiconductor processing. In marketing, ride-through flywheel systems can demand high prices because flywheel systems should have reliability and longevity advantages, and electrochemical batteries are inherently unsuitable and perform very poorly with repeated high power discharges.
The second category of flywheel systems, energy back-up, are used to provide power to support the load for the duration of a power interruption, until the utility power can be restored. Discharge times can be as much as 8 hours or more and the power levels are typically only a few kilowatts or less. Energy storage capacity though is large with multiple kilowatt-hours of storage. Promising applications for these systems are in telecommunications, for maintaining service reliability for telephone, cable TV, wireless and the Internet. Energy back-up flywheel uninterruptible power supplies are marketed based on their energy storage capacity, and because of the low power level, they compete with batteries primarily based on the increased longevity, higher reliability, and lower maintenance requirements. The more difficult cost targets for large energy back-up flywheel systems therefore make minimizing the cost per stored energy extremely important. The potential market for this application is enormous, so there has been considerable interest in developing flywheel energy back-up systems that would satisfy the industry requirements, all to no avail until now.
There are fundamentally two types of flywheel energy storage systems: low speed industrial steel flywheel systems and high speed composite flywheel systems. Commercial flywheel uninterruptible power supplies employing large steel flywheels currently operate with maximum tip speeds of only about 200 to 250 meters per second. The stored energy is proportional to the square of the tip speed and thus energy storage per flywheel size and weight is limited for flywheels with tip speed limited to 250 m/s. Strength and safety concerns have been factors that cause manufacturers to limit the operational speeds to 250 m/s or less. Small diameter steel flywheels can develop higher strengths due the fabrication attributes of the reduced size. For example, small diameter steel hubs for use inside composite energy storage flywheels have been laboratory tested to higher speeds. However to date, commercial operation of large diameter steel energy storage flywheels has been limited to relatively low speeds.
For efficiently storing large amounts of energy, especially in cost sensitive applications such as energy back-up, composite flywheels are commonly considered necessary. Composite flywheels can store large amounts of energy per weight due to the high strength capability of the constituent fibers such as glass and carbon. They can also be made of large diameter size while still having the maximum strength due to the strength being added by the already high strength fibers being wound into the rim. Composite flywheels have been very expensive in the past, however the price in recent years has been dramatically reduced due both to a drop in the price of carbon fiber and also the development of new more economical commercial processes.
Despite the benefits of increased energy storage capability with composite flywheels, they do have several undesirable traits. Composite materials outgass considerably due to the polymer matrix. The copious outgassing makes the maintainability of the vacuum in the surrounding enclosure difficult. Composite materials also experience a reduction in strength from the volumetric addition of resin with the high strength fibers and also some reduction occurs due to the ability of the matrix material, typically an epoxy, to translate the load from one fiber to others. A further reduction in strength results from the winding angle as fibers are wound onto the flywheel rim during manufacture and especially when multiple tows are used to make very large flywheels at low cost. When winding the fibers with the resin to form a composite flywheel rim, void flaws can be introduced into the parts. Despite these reductions in strength, composite material flywheels still have very high fiber direction strengths and for which the reductions can easily be accounted.
Unfortunately, composite material flywheels can exhibit some troublesome attributes that include poor temperature performance as well as creep and stress rupture, Most polymer matrix composite flywheels have low temperature capability, meaning that the epoxy matrix becomes soft at a relatively low temperature. The matrix loses its ability to optimally translate load between fibers with a relatively small increase in operating temperature. Because the radial strength is much lower than the hoop strength in filament wound flywheels, flywheels are usually constructed of multiple rims to mitigate radial stresses. One very common design approach to allow use of low cost thick flywheel rims is to use a glass/epoxy ring, with its lower modulus and higher density, inside a stiffer and lower density carbon fiber/epoxy ring. The glass ring grows with the larger radius carbon ring during rotation and thus avoids development of excessive radial tension. Unfortunately, over time and cyclic stress, the inner glass fibers creep and lose stiffness thereby causing the outer carbon fibers to carry unanticipated extra load. The outer carbon fibers also fatigue and lose strength with cycles. The end result is that a seemingly safe design, that would have had an initial benign radial crack failure, can have a lower speed catastrophic burst failure. Unlike a metal flywheel, where a failure results in pieces being projected radially outward during failure, a composite flywheel can fail exerting energetic fragments vertically or alternatively the fine radially directed fragments can be redirected vertically when they hit the container wall. Because most high energy flywheel systems are planned for shallow underground installation, the possibility of having a relatively uncontained vertical burst after years of operation is very undesirable.
To achieve a 20 year operating life with reliable service, the bearing system that supports the flywheel for rotation is also critically important. Such a bearing system preferably should be a non-contact bearing to preclude wear and not require lubrication, and should be protected from damaged during system shipping and installation. With use of magnetic bearings, it is desirable to have a simple construction and operation both for reduction of costs and also for reliable operation throughout the desired system life. Five active axes magnetic bearing systems are currently available, however to maintain stability, they require constant, very high frequency switching which can cause the amplifiers used to fail after a few years. These systems are also very complex and expensive.
The disclosed invention is a flywheel uninterruptible power supply capable of storing several kilowatt-hours of energy that achieves long life and higher reliability with significantly reduced costs. The flywheel of the invention is uniquely capable of storing large amounts of energy without using high strength composite materials. The flywheel is constructed of a solid steel cylinder that rotates with an internal stress and energy density more than twice that of previous steel flywheels used in commercial flywheel energy storage systems. The high rotational speed capability is the result of a combination of factors. The flywheel is solid, without a central hole, thereby reducing the hoop stress by 50% compared to a steel flywheel with an axial hole, and allowing equal and maximum hoop and radial stresses at the center. The flywheel is made in the shape of a cylinder and has a diameter dimension selected that enables the center of the flywheel to be subjected to a sufficient quench severity in the heat-treating process to develop high center hardness. The steel alloy used for manufacturing is chosen for deep hardenability, further increasing the cylinder centerline strength, and for its ability to generate high toughness. In one embodiment of the invention, the flywheel diameter is made less than the ideal critical diameter of the alloy steel such that the centerline of the flywheel can achieve 50% martensite structure for significantly improved mechanical properties.
The invention takes into account the specific use of flywheel systems in power quality applications, such as for protection against utility power interruptions. The actual number of full cycles that a flywheel would experience in a power back-up system lifetime is found to be unusually low for typical mechanical components. The number of full charge to full discharge cycles is likely substantially less than 2500 cycles after 20 years of use. Allowing for a substantial safety margin, the total number of such cycles can still be considered to be less than 10,000 cycles. A higher number of incomplete discharge cycles do occur but these have significantly less fatiguing effects. This fact is advantageously used to allow a further increase in the safe flywheel operating speed by employing nondestructive evaluation of the flywheel material to limit the maximum flaw size, in conjunction with a fracture mechanics analysis rated operating speed and use of high toughness steel alloy and heat treat conditions. In one embodiment, a cycle counter is used to prevent operation of the flywheel past its safe life rating.
Current steel flywheels operate at tip speeds of 250 meters per second and lower, storing less than 2 watt-hours per pound of flywheel. This invention allows operation at greater than 350 meters per second and up to roughly 550 meters per second, storing 4 to 10 watt-hours per pound of flywheel. Because of the higher density of steel than composites and the solid construction, the invention can also store more than twice the energy per outside volume of the containment vessel. The result is a significant reduction in the system size and amount of materials required. To operate at to such high speeds, the flywheel is supported using magnetic bearings at each end of the flywheel, preferably passive radial magnetic bearings as disclosed in international patent application no. PCT/US01/13951 filed on May 1, 2001 by Gabrys et al. and entitled xe2x80x9cFull Levitation Bearing System with Improved Passive Radial Magnetic Bearingsxe2x80x9d. The passive bearings, which can be of several configurations, reduce the amount of electronic control compared with full five active axes magnetic bearings and are simple, reliable and low cost. In one embodiment, the passive radial magnetic bearings are formed integrally in the axial faces of the flywheel cylinder to simplify construction while reducing costs. The active axial actuator for stabilizing the magnetic bearings can also be integrated into the cylinder face. Power consumption to levitate a flywheel weighing several hundred pounds can be less than 20 watts.
A preferred process for manufacture of the flywheel cylinders is described in which multiple cylindrical flywheels are manufactured from a single forged steel log. The log is quenched and tempered prior to cutting and machining into individual flywheels. This requires more difficult machining of already fully hardened flywheels, however it allows for efficient quality assurance steps to verify absence of flaws and centerline hardness and strength to insure safe operation. In one embodiment, the alloy steel is vacuum arc remelted prior to heat treating to eliminate impurities and promote a high level of uniformity in the cylinder.
The dimensions of the flywheel cylinder are bounded by upper and lower length dimensions for smooth operation. To prevent dynamic instability, the ratio of transverse to polar inertia is preferably greater than 1.2, however the length is preferably shorter than that which would cause encountering a flexural resonance of the cylinder in the operating speed range. By being solid and preferably without any flexural modes below the normal operating speed, the invention reduces the possibility of rotordynamic problems as such have been encountered with previous flywheel systems.
Further benefits of the invention""s capability to efficiently store large amounts of energy using a steel flywheel are achieved in the vacuum system that surrounds the flywheel for reduction of aerodynamic drag. The steel flywheel outgases less than 1% of the outgassing of polymer composite flywheels. This makes maintaining the vacuum much easier and less getter material is required to absorb the gases. The use of a cylindrical rotor instead of a disk also has the inadvertent advantage of reducing the cost of the vacuum container. Thinner container wall thickness are possible due to the smaller axial ends and hence smaller bending pressure load. In the event of a loss of vacuum, a composite flywheel would overheat and fail due to its poor thermal conductivity and very low temperature capability and high tip speed. The steel flywheel of the invention would simply slow down due to increased drag. The lower tip speed and higher thermal conductivity of the steel flywheel reduce the level of vacuum required.
The steel flywheel of this invention is in many ways safer than composite flywheels, commonly believed superior for storing large amounts of energy. Safety is increased by use of non-destructive inspection and the drastically increased rotor shock durability, absence of creep and temperature problems, and use of conventional well established materials, processes and testing. In the unlikely occurrence of a flywheel rotational failure, a steel flywheel classically fails in 3 pieces radially outward. The pieces are therefore directed into the soil for systems expectedly installed below ground. The steel flywheel is also completely recyclable and environmentally benign.